This invention relates to the video signal processing art. Specifically, apparatus is described for supplying two signal paths for filtering at least one of the signals without disturbing the relative phase relationship with another signal in a second signal path.
Video signal processing may require two or more signals to be amplitude filtered to different degrees while preserving the relative phase relationship between signals. After filtering is completed, the signals are combined. However, the phase of the signals combined is maintained in the same relationship as exists between the unfiltered signals.
An example of such a requirement can be found in the comb filter art as applied to television signal processing. Comb filters require that a video signal be delayed for a period of 1/Fh seconds where Fh is the horizontal scanning frequency. The delayed signal is combined with an undelayed video signal whereby a chroma signal is produced. The chroma signal is subtracted from an undelayed video signal to yield the luminance portion of the video signal.
Limitations in delay line bandwidth have made advantageous further filtering of the chroma signal before combining with the video signal to produce a luminance signal. Most of the chroma signal is located near 3.58 mHz and therefore, using a bandpass filter to further filter the chroma signal provides for mostly chroma signal which when subtractively combined with the video signal produces a substantially chroma free luminance signal.
This technique, however, requires that phase compensation be introduced into the undelayed video signal to compensate for a phase delay experienced by the chroma signal during bandpass filtering. The phase compensation delays the unfiltered video signal to the same degree, and over the same bandwidth, that the bandpass filtered signal is delayed. Phase matching two independent signal paths becomes very difficult over any appreciable bandwidth. Therefore, it is desirable to construct a network having two signal paths with common phase determining elements but having independent amplitude vs. frequency responses, the first path being a substantially all pass network and the second path having a bandpass filter structure. The signals produced by both signal paths are maintained in a fixed phase relationship but have a different amplitude relationship.